Solar cells are devices that convert light energy incident to its surface into electrical energy. Past efforts and ongoing research have led to improved efficiency by finding methods to convert more of the incident light energy into electrical energy. Several such improvements involve processes for roughening or texturing the surface of the solar cell substrate in order to reduce its reflective quality and therefore absorb more incident light for conversion. The texturing procedures improve the silicon solar efficiency by forming minute projections and recesses on the surface so that incident light is absorbed rather than reflected.
Various methods have been proposed in the prior art to achieve the desired textured silicon surface on a solar cell. The following patents describe some of the methods developed to texture a solar cell substrate.
U.S. Pat. No. 7,128,975 to Inomata entitled Multicrystalline Silicon Substrate and Process for Roughening Surface Thereof describes a process involving a wet etching step to form large irregularities followed by a dry etching step to form a multiplicity of fine textures over the large irregularities. The use of both wet etching and dry etching is a drawback
U.S. Pat. No. 6,752,897 to Jang et al. entitled Wet Etch System With Overflow Particle Removing Feature describes a wet etch system for removing particulate impurities from an etching liquid. The impurities are removed by causing a displacement of a significant amount of etchant into an overflow section of the tank by inserting a wafer carrier with a fixed volume into a full process tank. Fresh etchant is poured into the etch bath chamber prior to a subsequent etch cycle. The Jang patent does not recirculate used etchant.
U.S. Pat. No. 6,663,944 to Park et al. entitled Textured Semiconductor Wafer for Solar Cell describes a method for forming a plurality of random grooves on a surface of a semiconductor wafer. The grooves are formed by randomly depositing a protector on the surface and then dipping the wafer into an isotropic etching solution to etch the surface where the protector is not deposited. The need to deposit and later remove a protector layer adds extra steps.
U.S. Pat. Nos. 6,391,145 and 6,156,968 to Nishimoto et al. both entitled Solar Cell, a Method of Producing the Same and a Semiconductor Producing Apparatus describe a method whereby projections and recesses are uniformly formed on the surface of a silicon substrate by dipping in a wet etching solution. Spherical projections and recesses are formed on the surface of the substrate. The apparatus describes a multi tank configuration wherein one tank has recirculation of the etching solution and a means to maintain concentration of processed chemicals. The use of multiple tanks creates unneeded complexity.
U.S. Pat. No. 6,228,211 to Jeong entitled Apparatus for Etching a Glass Substrate describes an apparatus wherein a container includes a bubbling plate at the bottom thereof. The glass substrate to be etched is placed in the container where a liquid etchant is passed through holes in a porous plate to react with the surface of the substrate. Bubbles produced by the bubbling plate also pass through the holes in the porous plate to mix the liquid etchant and remove impurities from the surface of the glass substrate. The Jeong patent creates turbulent flow in the processing area and generates bubbles that can hinder the etching process.
U.S. Pat. No. 6,207,890 to Nakai et al. entitled Photovoltaic Element and Method for Manufacture Thereof describes an element that directly converts optical energy into electrical energy. The process involves forming many uneven sections on the surface of a silicon substrate then isotropically etching the surface. The bottoms of recessed sections are rounded and a p-type amorphous silicon layer is formed on the surface. The rounded bottom recesses allow an amorphous silicon layer to be deposited in a uniform thickness. Hydrogen bubbles created during the process attach to the substrate and prevent the alkali from reaching the surface at the point of attachment.
U.S. Pat. No. 4,980,017 to Kaji et al. entitled Method for Recirculating High-Temperature Etching Solution describes a method for recirculating a high temperature etching solution wherein the portion of an etching solution contained in an etching bath is continuously removed and a predetermined amount of pure water for adjusting the concentration of the etching solution is injected into the removed solution. The removed solution is then reheated to a predetermined temperature and recirculated into the etching bath. The described method does not produce a uniform flow rate or remove bubbles.
U.S. Pat. No. 4,252,865 to Gilbert et al. entitled Highly Solar-Energy Absorbing Device and Method of Making the Same uses sputtering to create a device wherein the surface of an amorphous semiconductor material has a particular characterization in that an array of outwardly projecting structural elements of relatively high aspect ratio and at effective lateral spacings on the order of magnitude of wave lengths within the solar energy spectrum.
U.S. Pat. No. 4,229,233 to Hansen et al. entitled Method for Fabricating Non-Reflective Semiconductor Surfaces by Anisotropic Reactive Ion Etching describes a process that takes place in a reactive ion etching tool, typically a diode configured system employing ambient gases which react with the silicon.
There are a number of drawbacks involved with the above-described methods and devices. Several of the processes utilize a dry etch or employ a wet etch system that does not involve a pump or recirculation of the etching solution. Other systems require the deposition of a protector on the surface to be etched. Still other processes describe the use of turbulent flow and a bottom tank inlet. In such prior art devices it is difficult to obtain a uniform flow rate across the semiconductor surface. Still other prior art patents describe an etching process that includes sputtering or a reactive ion etching chamber.
Accordingly, there is a need for an etching process and apparatus that enhances and speeds up the etching by providing more control over critical variables such as temperature uniformity, bubble removal and flow rate across the surface to be etched. In addition, a process is needed that produces laminar flow across the surface of the substrate. Further, a means of preventing attachment of bubbles or particles to the surface of the substrate is needed. The following invention provides these and other related advantages.